In an X-ray diffraction apparatus, a sample holder has a sample mounted on a pin extending a known distance from a cap that mates with a magnetized base on a goniometer. The sample is mechanically positioned in the center of an X-ray beam by a first movable arm which is located in a precise position relative to the goniometer base by a positioning mechanism and a mechanism that forces the pin into engagement with the first arm. The sample has a known height on the pin with respect to the cap and therefore, the sample can repeatedly be located in the center of the X-ray beam without the use of complex centering arrangements. In order to allow the sample holder to be removed from the goniometer base, a linkage is provided that releases the pin from the first arm.